Production of oxygen difluoride



Feb. 6, 1968 R. A. BROWN ET AL PRODUCTION OF OXYGEN DIFLUORIDE FiledAug. 17, 1965 ATTQRNEVY United States Patent Otitice 3,367,744 PatentedFeb. 6, 1968 3,367,744 PRODUCTION F OXYGEN DFLUORIDE Russell A. Brown,Meudham, NJ., and `loseph J. Ligi,

Baton Rouge, La., assignors to Allied Chemical Corporation, New York,N.Y., a corporation of New York Filed Aug. 17, 1965, Ser. No. 480,384 1Claim. (Cl. 23-20S) ABSTRACT GF THE DSCLSURE This application isconcerned with the preparation of oxygen diuoride by reaction ofliuorine gas with an aqueous solution of a base at a temperature notgreater than 70 F. and an aqueous liquid base to uorine gas volume ratioin excess of liters of aqueous base per liter of fluorine gas,

The present invention relates to the production of oxygen difluoride.More particularly, it is concerned with an improved continuous processfree of operational difiiculties whereby exceptionally high yields ofoxygen difluoride unrealized by prior art procedures are obtained.

In recent times, oxygen diluoride has attracted much attention as anessential ingredient in several high energy propellent and explosivesystems. Although various processes for preparation of oxygen diliuoridehave been reported in the literature, no practical commercial processfor production of oxygen diuoride has been developed to date.

The most commonly method for the preparation of oxygen diliuoride isbased on the reaction:

wherein M is an alkali metal such as sodium or potassium.

One conventional procedure for preparing oxygen ditluoride .inaccordance with this reaction involves a batch process wherein fluorinegas is flown concurrently with dilute caustic down a long narrow tube.In this procedure, excess quantities of fluorine, generally about 3 to 7liters of iluorine per liter of caustic, are disclosed to be ernployedunder practically all operating temperatures, generally ranging fromabout 70 to 105 F. The reaction is Said to be carried out using shortcontact times to avoid the secondary reaction:

Although oxygen difluoride yields up to about 70% have been reportedlyobtained by this procedure, extremely rapid increase in temperatureproduced during the highly exothermic reaction together with largequantities of evolved gases, i.e., unreacted iiuorine, oxygen diiiuorideproduct and oxygen produced by decomposition of oxygen diliuorideproduct, cause unpredictable destructive explosive reactions nottolerable in commercial operation.

Another procedure for oxygen dilluoride preparation in accordance withabove Reaction l comprises continuous ly generating excess quantities ofgaseous fluorine through a dilute caustic solution. A very short contacttime between the caustic and uorine is also required in this procedureaccording to prior art disclosures in order to avoid the above set forthsecondary reaction. The oxygen difluoride yield of the gaseous productobtained by this procedure is disclosed to be in the range of about 50percent. In addition to the low yields obtainable by this procedurenumerous operational problems are encountered when lluorine is reactedwith dilute caustic under the conditions of the procedure. For example,as in the batch process, rapid .increase in temperature resulting fromthe highly exothermic reaction not only is responsible for excessivedecomposition of oxygen diiiuoride usually resulting in yields of about50 percent but also causes explosions in the vapor phase of the reactorand in the recovery vessels due to the presence of excess quantities ofunreacted iiuorine. Furthermore, burning and high corrosion rates at theuorine nozzle outlet as well as at the liquid level within the reactordue to elevated caustic solution temperatures generated during thereaction and high oxygen concentrations resulting from excessivedecomposition of oxygen ditluoride are frequently experienced therebynecessitating expensive repairs to the equipment and disruptingcontinuous operation.

Therefore, there is a great need for development of a method capable ofproducing oxygen ditluoride in commercial quantities without theconcomitant disadvantageous characteristics of the prior art procedures.

With the foregoing in mind, the principal object of the presentinvention may be said to reside in the provision of a novel process forproducing oxygen diiiuoride by an economical and commercially feasiblemethod.

Another object of this invention resides in the provision of a processfor the production of oxygen dtluoride free ot explosion hazards.

A further object of this invention is to provide a continuous processfor the production of oxygen diuorde free of operational difficultiesdisruptive of continuous operation.

Further objects and advantages will be apparent from the followingdescription and drawing.

The drawing provides a simplified tlow diagram illustrative of the novelprocess to be described hereinafter.

According to the invention, oxygen diuon'de iS prepared in high yieldsby reaction of uorine gas with an aqueous solution of a base byintroducing the aqueous base into a reaction zone at a temperature notgreater than about 70 F., preferably not greater than about 45 F. anddispersing iiuorine gas into the aqueous base at a controlled ratesufficient to permit the aqueous base and uorine gas to be in contactfor a period of about 0.5 to 5.0 seconds, preferably l to 2 seconds,.and to provide a liquid to gas volume ratio in excess of 20 liters,preferably in excess of 50 liters, of aqueous base per liter of uorinegas. The contacting of uorine gas with the basic solution of the presentinvention is in sharp contrast to oxygen diiiuoride preparationprocedures disclosed in the prior art wherein short contact times,generally about 0.1 second, are employed. Furthermore, contrary to priorart procedures which require -short contact times to preclude excessivedecomposition of oxygen Cliuoride, the process of the present .inventionis based on the discovery that the specified liquid to gas ratios ratherthan short contact times of oxygen diiiuoride with basic solution areresponsible for controlling excessive decomposition oif oxygendiiluoride product produced in accordancc with Reaction l above.

While we do not wish to be bound by any specific theory, it is believedthat as oxygen diiluoride is formed in solution at an extremely highrate its heat of reaction causes rapid formation of small hot bubbles.Essentially, microscopic explosions occur in the reaction zone. Theoxygen diuoride formed undergoes rapid thermal decomposition andreaction with the basic solution in accordance with Reaction 2, aboveset forth. This decomposition reaction is greatly accelerated by theexistence of microscopic hot spots within the hot oxygen difluoride gasbubbles. By having present in the reaction zone a high basic solution tofluorine gas volume ratio, at least of the order specied, the rate ofheat removal from the bubbles is increased and the effect of themicroscopic hot spots which contribute to the excessive thermaldecomposition of oxygen diluoride is minimized. The decompositionreactions are therefore decreased and significant yield increases areobtained.

In accordance with the process of the present invention, oxygendifiuoride yields in excess of 70 percent of the theoretical arerealized. Moreover, when the temperature of the -basic solution employedis not greater than about 45 F. the lower temperatures around the oxygendiuoride bubbles slow the formative and decomposition Reactions l and 2set forth above, thereby reducing the rate of heat formation andallowing for obtainrnent of oxygen difluoride in yields in excess of 85percent of the theoretical. In addition, by conducting the reactionunder conditions of very high liquid to gas volume ratios it has beenfound that unreacted fiuorine is not obtained in the vapor phase of thereactor. Since rapid temperature increases are precluded in the reactionzone by the provision of a large heat sink in the form of the basicsolution, and unreacted uorine is not a product of the vapor phase ofthe reactor, potentially explosive gaseous mixtures normally found inthe procedures described in the prior art are avoided. Moreover, sincethe crude oxygen diuoride gaseous product obtained in the presentprocess is purer than crude gaseous products obtained by the prior artprocedures, less expensive recovery systems, accompanied by less oxygendiuoride purification losses, may be employed. Furthermore, burning atthe fiuorine nozzle is effectively suppressed by the procedure of thepresent invention, thereby allowing for continuity of operation anddecreased equipment replacement.

In practicing the invention, a dilute aqueous basic solution such as analkali metal hydroxide, illustratively sodium or potassium hydroxide,having a concentration of about 0.1 to 5.0 weight percent, preferably1.0 to 3.0 weight percent, based on the weight of solution, iscontinuously introduced into a reaction zone. The water employed informulating the dilute base may be obtained from local water suppliesbut preferably is demineralized by passage through any conventionalmixed bed anioncation ion exchange unit for reduction of the carbonatecontent contained therein. Prior to introduction into the reaction zonethe dilute base is preferably filtered for removal of impurities andthen cooled to a temperature below about 70 F., preferably below about45 F., by use of any conventional refrigeration system. The minimumtemperature at which the base may be introduced is limited only by theefiiciency of the refrigeration system employed and temperaturesapproximating the freezing point of dilute aqueous base at the specifiedconcentrations are not only suitable but provide a most effective methodfor increasing oxygen difluoride yields and minimizing operationaldiiculties of the nature above specified. The pressures employed in thereaction zone during the reaction are not critical and may range fromabout to l0 p.s.i.g. Slightly positive pressures in the range of about 0to 2 p.s.i.g. are generally preferred.

The rate of introduction and withdrawal of the basic solution into andfrom the reaction zone may be widely varied. In normal operation theconcentration of this reactant is maintained within the above specifiedrange during the course of reaction with the fiuorine gas. Since thebasic solution is employed not only as a reactant for reaction with theuorine gas but also serves as a heat sink for absorption of heatresulting from the highly exothermic reaction, sufiicient quantities ofbasic solution are maintained in the reactor and the reactor liquidcontents are continuously replenished with freshly precooled basicsolution to carry out these intended purposes. In practicing theinvention, the rate of introduction and withdrawal of the basic solutionis generally controlled to preclude an excessive increase in temperaturein the liquid phase of the reaction zone. Normally, it is desirable tomaintain the temperature of the liquid phase in the reactor within about25 F., preferably 10 F., of the temperature of the precooled basicsolution continuously fed into the reactor.

The gaseous fluorine reactant may be introduced into the reaction :oneat the opposite end from that at which the dilute base is charged thusproviding for countercurrent flow, or alternatively, may be introducedin the same direction of fiow at which the basic solution is charged. Inorder to insure adequate dissipation of the heat evolved from the highlyexothermic reaction and thereby preclude excessive thermal decompositionof the oxygen diuoride product, it has been found that the iiuorine gasmust be contacted with the aqueous base solution below the liquid levelthereof at a controlled rate to provide in the reaction zone a liquid togas volume ratio in excess of 20 liters, preferably in excess of 50liters, of basic solution per liter of fluorine gas. Advantageously, thefluorine gas is intimately dispersed in the aqeous basic solution sothat the very high liquid to gas volume ratio is continuouslymaintained, insuring substantially complete conversion of fiuorine tothe desired oxygen difluoride product. Intimate dispersion of thefluorine gas below the liquid level of the aqueous base solution may bereadily effected by use of any conventional apparatus capable ofdispersing the gaseous fiuorine reactant in finely divided bubble form.In general, the fluorine gas bubbles are in Contact with the aqueousbase below the liquid level thereof for a time sufficient to effectcomplete reaction of the uorine gas introduced into the reaction zone todesired oxygen diuoride product. Contact of the fluorine bubbles withthe aqueous base solution at these proportions of liquid to gas volumesnormally may range from about 0.5 to 5.0 seconds, although contact timesof 1.0 to 2.0 seconds are preferred. Such procedure is completelycontrary to prior art procedures wherein large quantities of unreactedfluorine gas are evolved into the vapor phase of the reaction zone andshort contact times are considered to be essential to avoid excessivedecomposition of ox gen diuoride when in prolonged contact with theaqueous base.

The rate at which the fiuorine gas is introduced into the reaction zoneis dependent upon the liquid to gas volume ratios present in thereaction zone. Generally, fluorine in dispersed bubble form isintroduced at a rate such that liquid to gas volume ratios aremaintained within the above specified range and the molar ratios of baseto fiuorine present in the reaction zone are in excess of 5 mols of`base (100% base basi-s) per Imol of fiuorine, and, preferably are inexcess of about 20 mols of base per mol of ftuorine charged.

Gxygen difluoride product, continuously formed as fiuorine is bubbledthrough the basic solution, is withdrawn from the vapor phase of thereactor and recovered by conventional procedures. In recovering oxygendifiuoride, the crude gases exiting from the reactor are fiown in seriesthrough a first trap operated at ambient temperatures and 0 p.s.i.g tol0 p.s.i.g to remove water mist and then through a second trap `cooledby Dry Ice at about F. and 0 l\p.s.i.g. to l0 p.s.i.\g. wherein watervapor is condensed from the crude product. The gaseous mixture resultingfrom the second trap, comprised primarily of oxygen difiuoride product,oxygen and minor amounts of carbon dioxide, is then condensed in aseries of cold traps cooled by liquid nitrogen at temperatures of about320 F. and pressures of 1 to 25 p.s.i.a. After each of the traps becomesfilled with the condensed product, oxygen Vis removed therefrom byslowly warming the trap, thereby allowing oxygen by-product to vaporize.Desired oxygen difluoride product in yields in excess of 70% oftheoretical and having a purity of at least 97% is normally recovered bythis procedure.

Applicants invention will now be described in more detail with relationto the drawing previously mentioned.

The process may be carried out by employing apparatus comprising anatmospheric blending tank 10 having inlet conduits 1l and 12 forintroducing concentrated base and water in the desired proportions. Thedilute base flows through valved conduit 14 to pump 15 which forces thedilute caustic through filter 17, precooler 21 and into reactor 22 viavalved conduits 15, 18 and 20. The iiow rate of the dilute base toreactor 22 is measured by flow meter 19 and control valve 23 is providedto control the instantaneous caustic ow rate. Temperature indicator 3Smeasures the temperature of the dilute base supplied to bubble reactor22.

Fluorine from any suitable source, preferably free of hydrogen uoride,flows through valved conduits 24 and 26 to reactor 22. Flow meter 25 ofany suitable type, such as a rotameter, is provided to measure theinstantaneous iluorine flow rate and valve 27 is provided to control theinstantaneous fluorine flow rate. If desired, liow meter 25 may be onethe standard iiow controllers available and it may be connected to asuitable type of control valve 27 in order to maintain automatically theinstantaneous Huorine flow rate at any predetermined value.

Fluorine is introduced into the dilute base through pipe (or pipes) 23provided with at least one orice. Spent caustic is continuouslywithdrawn from reactor 22 through conduit 33 and any absorbed gases arevented therefrom to the atmosphere via conduit 34. Thermometer 29measures the reaction temperature. This thermometer can be replaced byany standard temperature indicator controller which will partially orcompletely close valve 27 when a predetermined maximum temperature isreached in reactor 22 and/ or actuate valve 23 for the introduction ofcooled caustic and valve 30 for the withdrawal of spent caustic.Thermometer 31 measures the temperature of the gases evolved in reactor22 and thermometer 32 measures the temperature of the spent causticexiting through conduit 33.

The gases evolved from the reaction are removed from reactor 22 throughconduit 37 and passed to cold trap (or a series of cold traps) 38 forseparation of entrained water mist and water vapor. Pressure gauge 40 isprovided for determining the static pressure in the reactor, andpressure controller 36 is provided for the purpose of regulating thispressure. Gas analyzer 41 is provided in the system for purposes ofanalyzing the gaseous products resulting from reactor 22. The oxygendifiuoride product is then recovered from the gaseous mixture evolvedfrom reactor 22 by conventional procedures, for example, as abovedescribed.

It will be apparent to anyone skilled in the art that other types ofapparatus can be used equally successfully. The use of specificapparatus or combinations of specic apparatus is not critical to theprocess of this invention, but rather the procedural steps and processconditions are critical. Hence, any appara-tus which will provide theprocedural steps and `process conditions may be suitable for operatingthe process of this invention.

It is also apparent that the process of this invention can be operatedeither as a batch, intermittent batch or as a continuous process bymeans of recognizable variations inthe apparatus and the use thereof.

The process 'of this invention is illustrated by the following specicexamples.

Example 1 Into vertical corrosion resistant reactor 22 (1 x 11/2 X 30"),purged of -air with nitrogen, there was continuously introduced anaqueous potassium hydroxide solution having a concentration of 1.911% byweight based on the weight of solution and a -carbonate content of0.039% by weight (calculated as percent potassium carbonate). Thepotassium hydroxide solution, previously cooled to a temperature vof 42F., was fed into the reactor, maintained at a pressure of about 2p.s.i.g., at a rate of 640 grams per minute. 'Fluorine was continuouslybubbled into the aqueous potassium hydroxide solution through a 0.055inch (ID) pipe positioned 10 inches below the liquid level at a rate of700 cubic centimeters per minute. Spent potassium hydroxide solution ata temperature of 50 F. having a weight concentration of 1.46% wascontinuously withdrawn at a rate sufficient to maintain the liquid levelin the reactor. After operation for -a period of about 5 minutes, thetemperature of the potassium hydroxide solution in the reactor increasedto and was maintained at about 48 F. and the temperature of theresulting gases obtained from the reaction was about 78 F. Under theseconditions, bubbles of uorine gas were in contact with the cooledpotassium hydroxide solution for a period of about 0.5 to 2 seconds .andthe liquid to gas ratio in the liquid phase of the reaction zone wasabout 20 to 25 liters of potassium hydroxide solution per liter ofiiuorine gas.

The gases evolved from the vapor phase of the reactor were continuouslydischarged via conduit 37 and passed through cold trap 38 operated at-ll0 F. and 15 p.s.i.g. to remove water mist and water vapor. Analysisof the resulting cooled gases by gas chromatography showed that a yieldof 85.6% of theoretical of oxygen diiluoride was obtained.

Example 2 The procedure of Example 1 was repeated using a potassiumhydroxide solution having a concentration of 2.02% and a carbonatecontent of 0.024% (calculated as percent potassium carbonate). Thepotassium hydroxide solution was previously cooled to a temperature of72 F. and was introduced into the reactor, maintained at a pressure ofabout 2 p.s.i.g., at a rate of 640 grams per minute. Fluorine wascontinuously bubbled into the aqueous potassium hydroxide solutionthrough a 0.055 inch (ID) pipe positioned 20 inches below the liquidlevel at a rate of 700 cubic centimeters per minute. The spent potassiumhydroxide solution having a concentration of 1.53% exited at atemperature of 79 F. After a period of about 5 minutes the temperaturein the reactor increased to and was maintained at about 80 F. and thetemperature of the gases evolved from the reaction in the vapor phase ofthe reactor was about F. At these conditions, the liquid to gas volumeratio in the liquid phase of the reaction zone was about 25 to 30 litersof potassium hydroxide solution per liter of luorine gas and the contacttime between the liquid and fiuorine gas in the reactor was about l to 4seconds.

The eiiiuent gases from the reactor were passed through a cold trap asdescribed in Example 1 for removal of water mist and water Vapor.Analysis of the resulting cooled gases by gas chromatography showed thata yield of 74.53% of theoretical of oxygen diiluoride was obtained.

Example 3 The procedure of Example 1 was repeated using a potassiumhydroxide solution having a concentration of 2.11% by weight based onthe weight of solution and a carbonate content of about 0.03%(calculated as potassium carbonate). The potassium hydroxide solutionwas previously cooled to a temperature of 40 F. and was fed into thereactor, maintained at a pressure of about 2 p.s.i.g., at a rate of 640grams per minute. Fluorine was continuously bubbled into the aqueouspotassium hydroxide solution through a 0.1175 inch (ID) pipe positionedl0 inches below the liquid level at a rate of 700 cubic centimeters perminute. Spent potassium hydroxide solution having a concentration of1.69% by weight exited from the reactor at a temperature of 57 F. Aftera period of about 5 minutes, the temperature in the reactor increased toand was maintained at about 48 F. and the temperature of the gasesevolved in the vapor phase of the reactor was approximately 83 F. Atthese conditions, the liquid to gas volume ratio in the liquid phase ofthe reaction zone was about 20 to 25 liters of potassium hydroxidesolution per liter of lluorine gas and the contact time between theliquid and iluorine gas bubbles was about 0.5 to 2 seconds.

The eiuent gases from the reactor were passed through a cold trap asdescribed in Example 1 for removal of water mist and vapor. Analysis ofthe resulting cooled gases by gas chromatography showed that a yield of86.79% of theoretical of oxygen diuoride was obtained.

Example 4 Into a vertical corrosion resistant reactor 22, described inExample l, purged of air with nitrogen, there was continuouslyintroduced an aqueous potassium hydroxide solution having aconcentration of 2.0% based on the Weigh-t of solution. The potassiumhydroxide solution previously cooled to -a temperature of 70 F. was fedinto the reactor, maintained at a pressure of 2 p.s.i.g., at a rate of640 grams per minute. Oxygen difluoride gas, analyzing 99.19% Oxygendifluoride, 0.75% oxygen and 0.05% carbon dioxide by volume, wascontinuously bubbled .into the aqueous potassium hydroxide solutionthrough a 0.1175 inch (1D) pipe positioned 20 inches below the liquidlevel at a rate of 700 cubic centimeters per minute. Spent potassiumhydroxide solution having a concentra-tion of 1.97% exited from thereactor at a temperature of 70 F. After a period of about 5 minutes, thetemperature in the reactor was observed to be 71 F. and the temperatureof the gases in the vapor phase of the reactor was 70 F. Under theseconditions, oxygen diruoride bubbles were in contact with the potassiumhydroxide solution for a period of about 1 to 4 seconds. Analysis of thegases resulting from the vapor phase of the reactor by gaschromatography showed that the gaseous product consisted of 95.68%oxygen diuoride and 4.32% oxygen by volume.

This experiment illustrates that under the conditions of the presentinvention, substantial decomposition of oxygen diluoride product isavoided.

Although cer-tain preferred embodiments of the invention have beendisclosed for purpose of illustration, it will be evident that variouschanges may be made therein without departing from the scope and spiritof the invention.

We claim:

1. A continuous process for preparing oxygen diuoride which comprisescontinuously introducing into a reaction Zone at a tempera-ture greaterthan the freezing temperature of the aqueous base and not greater thanabout 45 F. a liquid aqueous solution consisting of potassium hydroxide,the concentration of which is .in the range of about 1 to 3% by weightbased on the Weight of the solution, continuously dispersing uorine gasbelow the liquid level of said potassium hydroxide solution at acontrolled rate Suiiicient to permit the aqueous base and fluorine gasto be in contact for a period of about 0.5 to 5 seconds and to provide aliquid to glas Volume ratio in excess of 50 liters of potassiumhydroxide solution per liter of luorine gas, continuously withdrawingspent potassium hydroxide solution from the liquid phase of the reactionZone at a rate sufficient to maintain the tempera- `ture of the liquidphase of the reaction Within about 10 F. of the potassium hydroxidesolution introduced into the reaction zone and to maintain said liquidto gas volume ratio and said potassium hydroxide concentration,continuously withdrawing gaseous e'liuent from the vapor phase of thereaction zone, and continuously recovering oxygen diliuoride productfrom the gaseous e'luent.

References Cited Schnizlein et al., 1. of Physical Chemistry, vol. 56,pp. 233 and 234, February 1952.

Streng, Chem. Rev., vol. 63 (6), pp. 608 and 610, December 1963.

OSCAR R. VERTIZ, Primary Examiner.

G. T. OZAK, Assistant Examiner.

